diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index d8b0786c0..b8b327c99 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -646,6 +646,18 @@
 #error "MBEDTLS_SSL_CID  defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_SSL_CID)            &&                 \
+    defined(MBEDTLS_SSL_CID_IN_LEN_MAX) &&                 \
+    MBEDTLS_SSL_CID_IN_LEN_MAX > 255
+#error "MBEDTLS_SSL_CID_IN_LEN_MAX too large (max 255)"
+#endif
+
+#if defined(MBEDTLS_SSL_CID)            &&                  \
+    defined(MBEDTLS_SSL_CID_OUT_LEN_MAX) &&                 \
+    MBEDTLS_SSL_CID_OUT_LEN_MAX > 255
+#error "MBEDTLS_SSL_CID_OUT_LEN_MAX too large (max 255)"
+#endif
+
 #if defined(MBEDTLS_SSL_DTLS_BADMAC_LIMIT) &&                              \
     ( !defined(MBEDTLS_SSL_TLS_C) || !defined(MBEDTLS_SSL_PROTO_DTLS) )
 #error "MBEDTLS_SSL_DTLS_BADMAC_LIMIT  defined, but not all prerequisites"
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index f40506565..8c5dd140b 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -386,6 +386,10 @@
 
 #define MBEDTLS_TLS_EXT_SESSION_TICKET              35
 
+/* The value of the CID extension is still TBD as of
+ * https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-04. */
+#define MBEDTLS_TLS_EXT_CID                        254 /* TBD */
+
 #define MBEDTLS_TLS_EXT_ECJPAKE_KKPP               256 /* experimental */
 
 #define MBEDTLS_TLS_EXT_RENEGOTIATION_INFO      0xFF01
@@ -1266,6 +1270,21 @@ struct mbedtls_ssl_context
     char own_verify_data[MBEDTLS_SSL_VERIFY_DATA_MAX_LEN]; /*!<  previous handshake verify data */
     char peer_verify_data[MBEDTLS_SSL_VERIFY_DATA_MAX_LEN]; /*!<  previous handshake verify data */
 #endif /* MBEDTLS_SSL_RENEGOTIATION */
+
+#if defined(MBEDTLS_SSL_CID)
+    /* CID configuration to use in subsequent handshakes. */
+
+    /*! The next incoming CID, chosen by the user and applying to
+     *  all subsequent handshakes. This may be different from the
+     *  CID currently used in case the user has re-configured the CID
+     *  after an initial handshake. */
+    unsigned char own_cid[ MBEDTLS_SSL_CID_IN_LEN_MAX ];
+    uint8_t own_cid_len;   /*!< The length of \c own_cid. */
+    uint8_t negotiate_cid; /*!< This indicates whether the CID extension should
+                            *   be negotiated in the next handshake or not.
+                            *   Possible values are #MBEDTLS_SSL_CID_ENABLED
+                            *   and #MBEDTLS_SSL_CID_DISABLED. */
+#endif /* MBEDTLS_SSL_CID */
 };
 
 #if defined(MBEDTLS_SSL_HW_RECORD_ACCEL)
@@ -1609,6 +1628,13 @@ int mbedtls_ssl_set_cid( mbedtls_ssl_context *ssl,
  *                     progress, this function will attempt to complete
  *                     the handshake first.
  *
+ * \note               If CID extensions have been exchanged but both client
+ *                     and server chose to use an empty CID, this function
+ *                     sets `*enabled` to #MBEDTLS_SSL_CID_DISABLED
+ *                     (the rationale for this is that the resulting
+ *                     communication is the same as if the CID extensions
+ *                     hadn't been used).
+ *
  * \return            \c 0 on success.
  * \return            A negative error code on failure.
  */
@@ -3135,7 +3161,7 @@ void mbedtls_ssl_conf_renegotiation_enforced( mbedtls_ssl_config *conf, int max_
  *                 (Default: 2^48 - 1)
  *
  *                 Renegotiation is automatically triggered when a record
- *                 counter (outgoing or ingoing) crosses the defined
+ *                 counter (outgoing or incoming) crosses the defined
  *                 threshold. The default value is meant to prevent the
  *                 connection from being closed when the counter is about to
  *                 reached its maximal value (it is not allowed to wrap).
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 9c4be53f7..e5da547c7 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -372,6 +372,18 @@ struct mbedtls_ssl_handshake_params
     unsigned char alt_out_ctr[8];       /*!<  Alternative record epoch/counter
                                               for resending messages         */
 
+#if defined(MBEDTLS_SSL_CID)
+    /* The state of CID configuration in this handshake. */
+
+    uint8_t cid_in_use; /*!< This indicates whether the use of the CID extension
+                         *   has been negotited. Possible values are
+                         *   #MBEDTLS_SSL_CID_ENABLED and
+                         *   #MBEDTLS_SSL_CID_DISABLED. */
+    unsigned char peer_cid[ MBEDTLS_SSL_CID_OUT_LEN_MAX ]; /*! The peer's CID */
+    uint8_t peer_cid_len;                                  /*!< The length of
+                                                            *   \c peer_cid.  */
+#endif /* MBEDTLS_SSL_CID */
+
     struct
     {
         size_t total_bytes_buffered; /*!< Cumulative size of heap allocated
@@ -597,6 +609,13 @@ struct mbedtls_ssl_transform
     mbedtls_cipher_context_t cipher_ctx_dec;    /*!<  decryption context      */
     int minor_ver;
 
+#if defined(MBEDTLS_SSL_CID)
+    uint8_t in_cid_len;
+    uint8_t out_cid_len;
+    unsigned char in_cid [ MBEDTLS_SSL_CID_OUT_LEN_MAX ];
+    unsigned char out_cid[ MBEDTLS_SSL_CID_OUT_LEN_MAX ];
+#endif /* MBEDTLS_SSL_CID */
+
     /*
      * Session specific compression layer
      */
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index aabf4d485..45f4c4047 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -475,6 +475,54 @@ static void ssl_write_ecjpake_kkpp_ext( mbedtls_ssl_context *ssl,
 }
 #endif /* MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */
 
+#if defined(MBEDTLS_SSL_CID)
+static void ssl_write_cid_ext( mbedtls_ssl_context *ssl,
+                               unsigned char *buf,
+                               size_t *olen )
+{
+    unsigned char *p = buf;
+    size_t ext_len;
+    const unsigned char *end = ssl->out_msg + MBEDTLS_SSL_OUT_CONTENT_LEN;
+
+    /*
+     * Quoting
+     * https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-04:
+     *
+     *   struct {
+     *      opaque cid<0..2^8-1>;
+     *   } ConnectionId;
+    */
+
+    *olen = 0;
+    if( ssl->conf->transport != MBEDTLS_SSL_TRANSPORT_DATAGRAM ||
+        ssl->negotiate_cid == MBEDTLS_SSL_CID_DISABLED )
+    {
+        return;
+    }
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "client hello, adding CID extension" ) );
+
+    /* ssl->own_cid_len is at most MBEDTLS_SSL_CID_IN_LEN_MAX
+     * which is at most 255, so the increment cannot overflow. */
+    if( end < p || (size_t)( end - p ) < (unsigned)( ssl->own_cid_len + 5 ) )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "buffer too small" ) );
+        return;
+    }
+
+    /* Add extension ID + size */
+    *p++ = (unsigned char)( ( MBEDTLS_TLS_EXT_CID >> 8 ) & 0xFF );
+    *p++ = (unsigned char)( ( MBEDTLS_TLS_EXT_CID      ) & 0xFF );
+    ext_len = (size_t) ssl->own_cid_len + 1;
+    *p++ = (unsigned char)( ( ext_len >> 8 ) & 0xFF );
+    *p++ = (unsigned char)( ( ext_len      ) & 0xFF );
+
+    *p++ = (uint8_t) ssl->own_cid_len;
+    memcpy( p, ssl->own_cid, ssl->own_cid_len );
+
+    *olen = ssl->own_cid_len + 5;
+}
+#endif /* MBEDTLS_SSL_CID */
+
 #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
 static void ssl_write_max_fragment_length_ext( mbedtls_ssl_context *ssl,
                                                unsigned char *buf,
@@ -1085,6 +1133,11 @@ static int ssl_write_client_hello( mbedtls_ssl_context *ssl )
     ext_len += olen;
 #endif
 
+#if defined(MBEDTLS_SSL_CID)
+    ssl_write_cid_ext( ssl, p + 2 + ext_len, &olen );
+    ext_len += olen;
+#endif /* MBEDTLS_SSL_CID */
+
 #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
     ssl_write_max_fragment_length_ext( ssl, p + 2 + ext_len, &olen );
     ext_len += olen;
@@ -1242,6 +1295,62 @@ static int ssl_parse_truncated_hmac_ext( mbedtls_ssl_context *ssl,
 }
 #endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
 
+#if defined(MBEDTLS_SSL_CID)
+static int ssl_parse_cid_ext( mbedtls_ssl_context *ssl,
+                              const unsigned char *buf,
+                              size_t len )
+{
+    size_t peer_cid_len;
+
+    if( /* CID extension only makes sense in DTLS */
+        ssl->conf->transport != MBEDTLS_SSL_TRANSPORT_DATAGRAM ||
+        /* The server must only send the CID extension if we have offered it. */
+        ssl->negotiate_cid == MBEDTLS_SSL_CID_DISABLED )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "CID extension unexpected" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                     MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
+        return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+    }
+
+    if( len == 0 )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "CID extension invalid" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                     MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
+        return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+    }
+
+    peer_cid_len = *buf++;
+    len--;
+
+    if( peer_cid_len > MBEDTLS_SSL_CID_OUT_LEN_MAX )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "CID extension invalid" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                     MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
+        return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+    }
+
+    if( len != peer_cid_len )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "CID extension invalid" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                     MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
+        return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+    }
+
+    ssl->handshake->cid_in_use = MBEDTLS_SSL_CID_ENABLED;
+    ssl->handshake->peer_cid_len = (uint8_t) peer_cid_len;
+    memcpy( ssl->handshake->peer_cid, buf, peer_cid_len );
+
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "Use of CID extension negotiated" ) );
+    MBEDTLS_SSL_DEBUG_BUF( 3, "Server CID", buf, peer_cid_len );
+
+    return( 0 );
+}
+#endif /* MBEDTLS_SSL_CID */
+
 #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
 static int ssl_parse_encrypt_then_mac_ext( mbedtls_ssl_context *ssl,
                                          const unsigned char *buf,
@@ -1893,6 +2002,20 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
             break;
 #endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
 
+#if defined(MBEDTLS_SSL_CID)
+        case MBEDTLS_TLS_EXT_CID:
+            MBEDTLS_SSL_DEBUG_MSG( 3, ( "found CID extension" ) );
+
+            if( ( ret = ssl_parse_cid_ext( ssl,
+                                           ext + 4,
+                                           ext_size ) ) != 0 )
+            {
+                return( ret );
+            }
+
+            break;
+#endif /* MBEDTLS_SSL_CID */
+
 #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
         case MBEDTLS_TLS_EXT_ENCRYPT_THEN_MAC:
             MBEDTLS_SSL_DEBUG_MSG( 3, ( "found encrypt_then_mac extension" ) );
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 33a38a652..51340b443 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -475,6 +475,78 @@ static int ssl_parse_max_fragment_length_ext( mbedtls_ssl_context *ssl,
 }
 #endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
 
+#if defined(MBEDTLS_SSL_CID)
+static int ssl_parse_cid_ext( mbedtls_ssl_context *ssl,
+                              const unsigned char *buf,
+                              size_t len )
+{
+    size_t peer_cid_len;
+
+    /* CID extension only makes sense in DTLS */
+    if( ssl->conf->transport != MBEDTLS_SSL_TRANSPORT_DATAGRAM )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
+        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
+    }
+
+    /*
+     * Quoting
+     * https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-04:
+     *
+     *   struct {
+     *      opaque cid<0..2^8-1>;
+     *   } ConnectionId;
+    */
+
+    if( len < 1 )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
+        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
+    }
+
+    peer_cid_len = *buf++;
+    len--;
+
+    if( len != peer_cid_len )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
+        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
+    }
+
+    /* Ignore CID if the user has disabled its use. */
+    if( ssl->negotiate_cid == MBEDTLS_SSL_CID_DISABLED )
+    {
+        /* Leave ssl->handshake->cid_in_use in its default
+         * value of MBEDTLS_SSL_CID_DISABLED. */
+        MBEDTLS_SSL_DEBUG_MSG( 3, ( "Client sent CID extension, but CID disabled" ) );
+        return( 0 );
+    }
+
+    if( peer_cid_len > MBEDTLS_SSL_CID_OUT_LEN_MAX )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
+        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
+    }
+
+    ssl->handshake->cid_in_use = MBEDTLS_SSL_CID_ENABLED;
+    ssl->handshake->peer_cid_len = (uint8_t) peer_cid_len;
+    memcpy( ssl->handshake->peer_cid, buf, peer_cid_len );
+
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "Use of CID extension negotiated" ) );
+    MBEDTLS_SSL_DEBUG_BUF( 3, "Client CID", buf, peer_cid_len );
+
+    return( 0 );
+}
+#endif /* MBEDTLS_SSL_CID */
+
 #if defined(MBEDTLS_SSL_TRUNCATED_HMAC)
 static int ssl_parse_truncated_hmac_ext( mbedtls_ssl_context *ssl,
                                          const unsigned char *buf,
@@ -1823,6 +1895,16 @@ read_record_header:
                 break;
 #endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
 
+#if defined(MBEDTLS_SSL_CID)
+            case MBEDTLS_TLS_EXT_CID:
+                MBEDTLS_SSL_DEBUG_MSG( 3, ( "found CID extension" ) );
+
+                ret = ssl_parse_cid_ext( ssl, ext + 4, ext_size );
+                if( ret != 0 )
+                    return( ret );
+                break;
+#endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
+
 #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
             case MBEDTLS_TLS_EXT_ENCRYPT_THEN_MAC:
                 MBEDTLS_SSL_DEBUG_MSG( 3, ( "found encrypt then mac extension" ) );
@@ -2100,6 +2182,54 @@ static void ssl_write_truncated_hmac_ext( mbedtls_ssl_context *ssl,
 }
 #endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
 
+#if defined(MBEDTLS_SSL_CID)
+static void ssl_write_cid_ext( mbedtls_ssl_context *ssl,
+                               unsigned char *buf,
+                               size_t *olen )
+{
+    unsigned char *p = buf;
+    size_t ext_len;
+    const unsigned char *end = ssl->out_msg + MBEDTLS_SSL_OUT_CONTENT_LEN;
+
+    *olen = 0;
+
+    /* Skip writing the extension if we don't want to use it or if
+     * the client hasn't offered it. */
+    if( ssl->handshake->cid_in_use == MBEDTLS_SSL_CID_DISABLED )
+        return;
+
+    /* ssl->own_cid_len is at most MBEDTLS_SSL_CID_IN_LEN_MAX
+     * which is at most 255, so the increment cannot overflow. */
+    if( end < p || (size_t)( end - p ) < (unsigned)( ssl->own_cid_len + 5 ) )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "buffer too small" ) );
+        return;
+    }
+
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, adding CID extension" ) );
+
+    /*
+     * Quoting
+     * https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-04:
+     *
+     *   struct {
+     *      opaque cid<0..2^8-1>;
+     *   } ConnectionId;
+    */
+
+    *p++ = (unsigned char)( ( MBEDTLS_TLS_EXT_CID >> 8 ) & 0xFF );
+    *p++ = (unsigned char)( ( MBEDTLS_TLS_EXT_CID      ) & 0xFF );
+    ext_len = (size_t) ssl->own_cid_len + 1;
+    *p++ = (unsigned char)( ( ext_len >> 8 ) & 0xFF );
+    *p++ = (unsigned char)( ( ext_len      ) & 0xFF );
+
+    *p++ = (uint8_t) ssl->own_cid_len;
+    memcpy( p, ssl->own_cid, ssl->own_cid_len );
+
+    *olen = ssl->own_cid_len + 5;
+}
+#endif /* MBEDTLS_SSL_CID */
+
 #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
 static void ssl_write_encrypt_then_mac_ext( mbedtls_ssl_context *ssl,
                                             unsigned char *buf,
@@ -2621,6 +2751,11 @@ static int ssl_write_server_hello( mbedtls_ssl_context *ssl )
     ext_len += olen;
 #endif
 
+#if defined(MBEDTLS_SSL_CID)
+    ssl_write_cid_ext( ssl, p + 2 + ext_len, &olen );
+    ext_len += olen;
+#endif
+
 #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
     ssl_write_encrypt_then_mac_ext( ssl, p + 2 + ext_len, &olen );
     ext_len += olen;
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 50464751c..4d41d8328 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -121,34 +121,66 @@ static void ssl_update_in_pointers( mbedtls_ssl_context *ssl,
 #if defined(MBEDTLS_SSL_CID)
 /* Top-level Connection ID API */
 
-/* WARNING: This implementation is a stub and doesn't do anything!
- *          It is included solely to allow review and coding against
- *          the new Connection CID API. */
+/* WARNING: The CID feature isn't fully implemented yet
+ *          and will not be used. */
 int mbedtls_ssl_set_cid( mbedtls_ssl_context *ssl,
                          int enable,
                          unsigned char const *own_cid,
                          size_t own_cid_len )
 {
-    ((void) ssl);
-    ((void) enable);
-    ((void) own_cid);
-    ((void) own_cid_len);
+    ssl->negotiate_cid = enable;
+    if( enable == MBEDTLS_SSL_CID_DISABLED )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 3, ( "Disable use of CID extension." ) );
+        return( 0 );
+    }
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "Enable use of CID extension." ) );
+
+    if( own_cid_len > MBEDTLS_SSL_CID_IN_LEN_MAX )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 3, ( "CID too large: Maximum %u, actual %u",
+                                    (unsigned) MBEDTLS_SSL_CID_IN_LEN_MAX,
+                                    (unsigned) own_cid_len ) );
+        return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
+    }
+
+    memcpy( ssl->own_cid, own_cid, own_cid_len );
+    /* Truncation is not an issue here because
+     * MBEDTLS_SSL_CID_IN_LEN_MAX at most 255. */
+    ssl->own_cid_len = (uint8_t) own_cid_len;
+
+    MBEDTLS_SSL_DEBUG_BUF( 3, "Own CID", own_cid, own_cid_len );
     return( 0 );
 }
 
-/* WARNING: This implementation is a stub and doesn't do anything!
- *          It is included solely to allow review and coding against
- *          the new Connection CID API. */
+/* WARNING: The CID feature isn't fully implemented yet
+ *          and will not be used. */
 int mbedtls_ssl_get_peer_cid( mbedtls_ssl_context *ssl,
                      int *enabled,
                      unsigned char peer_cid[ MBEDTLS_SSL_CID_OUT_LEN_MAX ],
                      size_t *peer_cid_len )
 {
-    ((void) ssl);
-    ((void) peer_cid);
-    ((void) peer_cid_len);
-
     *enabled = MBEDTLS_SSL_CID_DISABLED;
+
+    if( ssl->state != MBEDTLS_SSL_HANDSHAKE_OVER )
+        return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
+
+    /* We report MBEDTLS_SSL_CID_DISABLED in case the CID extensions
+     * were used, but client and server requested the empty CID.
+     * This is indistinguishable from not using the CID extension
+     * in the first place. */
+    if( ssl->transform_in->in_cid_len  == 0 &&
+        ssl->transform_in->out_cid_len == 0 )
+    {
+        return( 0 );
+    }
+
+    *peer_cid_len = ssl->transform_in->out_cid_len;
+    memcpy( peer_cid, ssl->transform_in->out_cid,
+            ssl->transform_in->out_cid_len );
+
+    *enabled = MBEDTLS_SSL_CID_ENABLED;
+
     return( 0 );
 }
 #endif /* MBEDTLS_SSL_CID */
@@ -918,6 +950,25 @@ int mbedtls_ssl_derive_keys( mbedtls_ssl_context *ssl )
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
     }
 
+#if defined(MBEDTLS_SSL_CID)
+    /* Copy own and peer's CID if the use of the CID
+     * extension has been negotiated. */
+    if( ssl->handshake->cid_in_use == MBEDTLS_SSL_CID_ENABLED )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 3, ( "Copy CIDs into SSL transform" ) );
+        transform->in_cid_len = ssl->own_cid_len;
+        transform->out_cid_len = ssl->handshake->peer_cid_len;
+        memcpy( transform->in_cid, ssl->own_cid, ssl->own_cid_len );
+        memcpy( transform->out_cid, ssl->handshake->peer_cid,
+                ssl->handshake->peer_cid_len );
+
+        MBEDTLS_SSL_DEBUG_BUF( 3, "Outgoing CID", transform->out_cid,
+                               transform->out_cid_len );
+        MBEDTLS_SSL_DEBUG_BUF( 3, "Incoming CID", transform->in_cid,
+                               transform->in_cid_len );
+    }
+#endif /* MBEDTLS_SSL_CID */
+
     /*
      * Set appropriate PRF function and other SSL / TLS / TLS1.2 functions
      */
diff --git a/programs/ssl/ssl_client2.c b/programs/ssl/ssl_client2.c
index a524f81ce..cb5bea7dc 100644
--- a/programs/ssl/ssl_client2.c
+++ b/programs/ssl/ssl_client2.c
@@ -2180,7 +2180,7 @@ int main( int argc, char *argv[] )
                             (unsigned) peer_cid_len );
             while( idx < peer_cid_len )
             {
-                mbedtls_printf( "%#02x ", peer_cid[ idx ] );
+                mbedtls_printf( "%02x ", peer_cid[ idx ] );
                 idx++;
             }
             mbedtls_printf( "\n" );
diff --git a/programs/ssl/ssl_server2.c b/programs/ssl/ssl_server2.c
index 60c5a507d..a94ddac8e 100644
--- a/programs/ssl/ssl_server2.c
+++ b/programs/ssl/ssl_server2.c
@@ -924,7 +924,8 @@ int sni_callback( void *p_info, mbedtls_ssl_context *ssl,
 
 #endif /* SNI_OPTION */
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED) || \
+    defined(MBEDTLS_SSL_CID)
 
 #define HEX2NUM( c )                        \
     do                                      \
@@ -967,6 +968,10 @@ int unhexify( unsigned char *output, const char *input, size_t *olen )
     return( 0 );
 }
 
+#endif
+
+#if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED)
+
 typedef struct _psk_entry psk_entry;
 
 struct _psk_entry
@@ -3330,7 +3335,7 @@ handshake:
                             (unsigned) peer_cid_len );
             while( idx < peer_cid_len )
             {
-                mbedtls_printf( "%#02x ", peer_cid[ idx ] );
+                mbedtls_printf( "%02x ", peer_cid[ idx ] );
                 idx++;
             }
             mbedtls_printf( "\n" );
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 0037429a9..de0653241 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -1278,94 +1278,289 @@ run_test    "Truncated HMAC, DTLS: client enabled, server enabled" \
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client enabled, server disabled" \
-            "$P_SRV dtls=1 cid=0" \
-            "$P_CLI dtls=1 cid=1 cid_val=deadbeef" \
-             0
+            "$P_SRV debug_level=3 dtls=1 cid=0" \
+            "$P_CLI debug_level=3 dtls=1 cid=1 cid_val=deadbeef" \
+            0 \
+            -s "Disable use of CID extension." \
+            -s "found CID extension"           \
+            -s "Client sent CID extension, but CID disabled" \
+            -c "Enable use of CID extension."  \
+            -c "client hello, adding CID extension" \
+            -S "server hello, adding CID extension" \
+            -C "found CID extension" \
+            -S "Copy CIDs into SSL transform" \
+            -C "Copy CIDs into SSL transform" \
+            -c "Use of Connection ID was rejected by the server"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client disabled, server enabled" \
-            "$P_SRV dtls=1 cid=1 cid_val=deadbeef" \
-            "$P_CLI dtls=1 cid=0" \
-             0
+            "$P_SRV debug_level=3 dtls=1 cid=1 cid_val=deadbeef" \
+            "$P_CLI debug_level=3 dtls=1 cid=0" \
+            0 \
+            -c "Disable use of CID extension." \
+            -C "client hello, adding CID extension"           \
+            -S "found CID extension"           \
+            -s "Enable use of CID extension." \
+            -S "server hello, adding CID extension" \
+            -C "found CID extension" \
+            -S "Copy CIDs into SSL transform" \
+            -C "Copy CIDs into SSL transform"  \
+            -s "Use of Connection ID was not offered by the client"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client+Server enabled, Client+Server CID nonempty" \
-            "$P_SRV dtls=1 cid=1 cid_val=dead" \
-            "$P_CLI dtls=1 cid=1 cid_val=beef" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1 cid_val=dead" \
+            "$P_CLI debug_level=3 dtls=1 cid=1 cid_val=beef" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -s "Use of Connection ID has been negotiated" \
+            -c "Use of Connection ID has been negotiated" \
+            -c "Peer CID (length 2 Bytes): de ad" \
+            -s "Peer CID (length 2 Bytes): be ef"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client+Server enabled, Client CID empty" \
-            "$P_SRV dtls=1 cid=1 cid_val=deadbeef" \
-            "$P_CLI dtls=1 cid=1" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1 cid_val=deadbeef" \
+            "$P_CLI debug_level=3 dtls=1 cid=1" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -s "Use of Connection ID has been negotiated" \
+            -c "Use of Connection ID has been negotiated" \
+            -c "Peer CID (length 4 Bytes): de ad be ef" \
+            -s "Peer CID (length 0 Bytes):"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client+Server enabled, Server CID empty" \
-            "$P_SRV dtls=1 cid=1" \
-            "$P_CLI dtls=1 cid=1 cid_val=deadbeef" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1" \
+            "$P_CLI debug_level=3 dtls=1 cid=1 cid_val=deadbeef" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -s "Use of Connection ID has been negotiated" \
+            -c "Use of Connection ID has been negotiated" \
+            -s "Peer CID (length 4 Bytes): de ad be ef" \
+            -c "Peer CID (length 0 Bytes):"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client+Server enabled, Client+Server CID empty" \
-            "$P_SRV dtls=1 cid=1" \
-            "$P_CLI dtls=1 cid=1" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1" \
+            "$P_CLI debug_level=3 dtls=1 cid=1" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -S "Use of Connection ID has been negotiated" \
+            -C "Use of Connection ID has been negotiated"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client+Server enabled, Client+Server CID nonempty, AES-128-CCM-8" \
-            "$P_SRV dtls=1 cid=1 cid_val=dead" \
-            "$P_CLI dtls=1 cid=1 cid_val=beef force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CCM-8" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1 cid_val=dead" \
+            "$P_CLI debug_level=3 dtls=1 cid=1 cid_val=beef force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CCM-8" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -s "Use of Connection ID has been negotiated" \
+            -c "Use of Connection ID has been negotiated" \
+            -c "Peer CID (length 2 Bytes): de ad" \
+            -s "Peer CID (length 2 Bytes): be ef"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client+Server enabled, Client CID empty, AES-128-CCM-8" \
-            "$P_SRV dtls=1 cid=1 cid_val=deadbeef" \
-            "$P_CLI dtls=1 cid=1 force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CCM-8" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1 cid_val=deadbeef" \
+            "$P_CLI debug_level=3 dtls=1 cid=1 force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CCM-8" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -s "Use of Connection ID has been negotiated" \
+            -c "Use of Connection ID has been negotiated" \
+            -c "Peer CID (length 4 Bytes): de ad be ef" \
+            -s "Peer CID (length 0 Bytes):"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client+Server enabled, Server CID empty, AES-128-CCM-8" \
-            "$P_SRV dtls=1 cid=1" \
-            "$P_CLI dtls=1 cid=1 cid_val=deadbeef force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CCM-8" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1" \
+            "$P_CLI debug_level=3 dtls=1 cid=1 cid_val=deadbeef force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CCM-8" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -s "Use of Connection ID has been negotiated" \
+            -c "Use of Connection ID has been negotiated" \
+            -s "Peer CID (length 4 Bytes): de ad be ef" \
+            -c "Peer CID (length 0 Bytes):"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client+Server enabled, Client+Server CID empty, AES-128-CCM-8" \
-            "$P_SRV dtls=1 cid=1" \
-            "$P_CLI dtls=1 cid=1 force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CCM-8" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1" \
+            "$P_CLI debug_level=3 dtls=1 cid=1 force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CCM-8" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -S "Use of Connection ID has been negotiated" \
+            -C "Use of Connection ID has been negotiated"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client+Server enabled, Client+Server CID nonempty, AES-128-CBC" \
-            "$P_SRV dtls=1 cid=1 cid_val=dead" \
-            "$P_CLI dtls=1 cid=1 cid_val=beef force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CBC-SHA256" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1 cid_val=dead" \
+            "$P_CLI debug_level=3 dtls=1 cid=1 cid_val=beef force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CBC-SHA256" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -s "Use of Connection ID has been negotiated" \
+            -c "Use of Connection ID has been negotiated" \
+            -c "Peer CID (length 2 Bytes): de ad" \
+            -s "Peer CID (length 2 Bytes): be ef"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client+Server enabled, Client CID empty, AES-128-CBC" \
-            "$P_SRV dtls=1 cid=1 cid_val=deadbeef" \
-            "$P_CLI dtls=1 cid=1 force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CBC-SHA256" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1 cid_val=deadbeef" \
+            "$P_CLI debug_level=3 dtls=1 cid=1 force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CBC-SHA256" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -s "Use of Connection ID has been negotiated" \
+            -c "Use of Connection ID has been negotiated" \
+            -c "Peer CID (length 4 Bytes): de ad be ef" \
+            -s "Peer CID (length 0 Bytes):"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client+Server enabled, Server CID empty, AES-128-CBC" \
-            "$P_SRV dtls=1 cid=1" \
-            "$P_CLI dtls=1 cid=1 cid_val=deadbeef force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CBC-SHA256" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1" \
+            "$P_CLI debug_level=3 dtls=1 cid=1 cid_val=deadbeef force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CBC-SHA256" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -s "Use of Connection ID has been negotiated" \
+            -c "Use of Connection ID has been negotiated" \
+            -s "Peer CID (length 4 Bytes): de ad be ef" \
+            -c "Peer CID (length 0 Bytes):"
 
 requires_config_enabled MBEDTLS_SSL_CID
 run_test    "(STUB) Connection ID: Client+Server enabled, Client+Server CID empty, AES-128-CBC" \
-            "$P_SRV dtls=1 cid=1" \
-            "$P_CLI dtls=1 cid=1 force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CBC-SHA256" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1" \
+            "$P_CLI debug_level=3 dtls=1 cid=1 force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CBC-SHA256" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -S "Use of Connection ID has been negotiated" \
+            -C "Use of Connection ID has been negotiated"
 
 requires_config_enabled MBEDTLS_SSL_CID
 requires_config_enabled MBEDTLS_SSL_RENEGOTIATION
 run_test    "(STUB) Connection ID: Client+Server enabled, renegotiate" \
-            "$P_SRV dtls=1 cid=1 cid_val=dead renegotiation=1" \
-            "$P_CLI dtls=1 cid=1 cid_val=beef renegotiation=1 renegotiate=1" \
-            0
+            "$P_SRV debug_level=3 dtls=1 cid=1 cid_val=dead renegotiation=1" \
+            "$P_CLI debug_level=3 dtls=1 cid=1 cid_val=beef renegotiation=1 renegotiate=1" \
+            0 \
+            -c "Enable use of CID extension." \
+            -s "Enable use of CID extension." \
+            -c "client hello, adding CID extension" \
+            -s "found CID extension"           \
+            -s "Use of CID extension negotiated" \
+            -s "server hello, adding CID extension" \
+            -c "found CID extension" \
+            -c "Use of CID extension negotiated" \
+            -s "Copy CIDs into SSL transform" \
+            -c "Copy CIDs into SSL transform" \
+            -s "Use of Connection ID has been negotiated" \
+            -c "Use of Connection ID has been negotiated" \
+            -c "Peer CID (length 2 Bytes): de ad" \
+            -s "Peer CID (length 2 Bytes): be ef"
 
 # Tests for Encrypt-then-MAC extension